Magnetic resonance imaging may be used in conjunction with ultrasound focusing in a variety of medical applications. Ultrasound penetrates well through soft tissues and, due to its short wavelengths, can be focused to spots with dimensions of a few millimeters. As a consequence of these properties, ultrasound can be and has been used for various diagnostic and therapeutic medical purposes, including ultrasound imaging and non-invasive surgery. For example, focused ultrasound may be used to ablate diseased (e.g., cancerous) tissue without causing significant damage to surrounding healthy tissue. An ultrasound focusing system generally utilizes an acoustic transducer surface, or an array of transducer surfaces, to generate an ultrasound beam. In transducer arrays, the individual surfaces, or “elements,” are typically individually controllable—i.e., their vibration phases and/or amplitudes can be set independently of one another—allowing the beam to be steered in a desired direction and focused at a desired distance. The ultrasound system often also includes receiving elements, integrated into the transducer array or provided in form of a separate detector, that help monitor the focused ultrasound treatment, primarily for safety purposes. For example, the receiving elements may serve to detect ultrasound reflected off interfaces between the transducer and the target tissue, which may result from air bubbles on the skin that need to be removed to avoid skin burns. The receiving elements may also be used to detect cavitation in overheated tissues (i.e., the formation of cavities due to the collapse of bubbles formed in the liquid of the tissue).
To visualize the target tissue and guide the ultrasound focus during therapy, magnetic resonance imaging may be used. In brief, MRI involves placing a subject, such as the patient, into a homogeneous static magnetic field, thus aligning the spins of hydrogen nuclei in the tissue. Then, by applying a radio-frequency (RF) electromagnetic pulse of the right frequency (the “resonance frequency”), the spins may be flipped, temporarily destroying the alignment and inducing a response signal. Different tissues produce different response signals, resulting in a contrast among theses tissues in MR images. Because the resonance frequency and the frequency of the response signal depend on the magnetic field strength, the origin and frequency of the response signal can be controlled by superposing magnetic gradient fields onto the homogeneous field to render the field strength dependent on position. By using time-variable gradient fields, MRI “scans” of the tissue can be obtained. Many MRI protocols utilize time-dependent gradients in two or three mutually perpendicular directions. The relative strengths and timing of the gradient fields and RF pulses are specified in a pulse sequence and may be illustrated in a pulse sequence diagram.
Time-dependent magnetic field gradients may be exploited, in combination with the tissue dependence of the MRI response signal, to visualize, for example, a brain tumor, and determine its location relative to the patient's skull. An ultrasound transducer system, such as an array of transducers attached to a housing, may then be placed on the patient's head. The ultrasound transducer may include MR tracking coils or other markers that enable determining its position and orientation relative to the target tissue in the MR image. Based on computations of the required transducer element phases and amplitudes, the transducer array is then driven so as to focus ultrasound into the tumor. Alternatively or additionally, the ultrasound focus itself may be visualized, using a technique such as thermal MRI or acoustic resonance force imaging (ARFI), and such measurement of the focus location may be used to adjust the focus position. These methods are generally referred to as magnetic-resonance-guided focusing of ultrasound (MRgFUS).
In addition, an MRI apparatus and an ultrasound imaging system may be combined to offer the strengths of both imaging modalities and thereby provide novel insights into the morphology and function of normal and diseased tissues. MRI is used widely for both diagnostic and therapeutic applications because of its multi-planar imaging capability, high signal-to-noise ratio, and sensitivity to subtle changes in soft tissue morphology and function. Ultrasound imaging, on the other hand, has advantages including high temporal resolution, high sensitivity to acoustic scatters (such as calcifications and gas bubbles), excellent visualization, and measurement of blood flow, low cost, and portability. Benefits of combining these complementary modalities have been shown in intraoperative neurosurgical applications and breast biopsy guidance. By performing imaging with both modalities simultaneously, a number of issues such as spatial and temporal registration between data sets may be simplified. In addition, measurements of unique physiological parameters can be made with each modality to fully characterize the organ or tissue under evolution.
The simultaneous operation of ultrasound and MRI apparatus, however, can lead to undesired interferences. For example, MRI is very sensitive to RF noise generated by the focused ultrasound system (see, e.g., U.S. Pat. No. 6,735,461). Conversely, focused ultrasound procedures often involve RF-sensitive operations (such as the ultrasound detection that may accompany treatment with focused ultrasound) that are easily disturbed by RF excitation signals and/or time-varying field gradient generated by the MRI system. Prior-art approaches to avoiding such interference typically involve shielding. Shielding the ultrasound system from interfering MR signals typically requires covering or surrounding the whole transducer and associated cables in metallic shield. In some systems, however, acoustic constraints prevent complete encapsulation of the ultrasound-receiving elements, resulting in penetration of, e.g., the front layer of a receiver and/or the cables by some amount of RF noise. Accordingly, there is a need for alternative approaches in MRgFUS applications to minimize or avoid interferences between the two systems.